Drive circuit for fluid motor

Abstract

When the load acting on a fluid motor ( 25 ) increases, a changeover valve ( 61 ) starts to be changed over toward a low-speed position L, but at this time an additional pilot fluid whose pressure is determined by the amount of fluid flowing into or out of a second passage ( 68 ) through first and second notches functions as a pressure regulator for a spool ( 65 ), and lowers the pressure within the second passage ( 68 ) without lowering the pressure within a high pressure-side main circuit ( 28 ), thereby tilting a swash plate ( 26 ) from a position of high-speed rotation to a position of low-speed rotation.

What is claimed is:  
     
       1. A drive circuit for a fluid motor comprising: a fluid motor having a swash plate which is tiltable between positions of high-speed and low-speed rotation; a tilting piston which, when a high-pressure fluid is introduced thereto, pushes and tilts said swash plate and tilts it to the position of high-speed rotation, and which, when set under a tank pressure, allows said swash plate to tilt to the position of low-speed rotation; a changeover valve which is interposed midway in a connecting passage for connecting said tilting piston and a selector valve for selectively obtaining the high-pressure fluid from two main circuits of said fluid motor, and which is changed over between a high-speed position for introducing to said tilting piston the high-pressure fluid obtained by said selector valve and a low-speed position for discharging the fluid acting on said tilting piston to a drain passage; a first pilot passage for introducing into said changeover valve a constant-pressure pilot fluid of a direction in which said changeover valve is changed over to the high-speed position; and a second pilot passage for introducing into said changeover valve the high-pressure fluid, selectively obtained from said main circuits, of a direction in which the changeover valve is changed over to the low-speed position, said drive circuit for a fluid motor further comprising: 
       an additional pilot passage ( 74 ) for introducing, from a portion of said connecting passage between said changeover valve and said tilting piston into said changeover valve, an additional pilot fluid of the direction in which said changeover valve is changed over to the low-speed position, and that  
       a first narrow passage ( 81 ,  65   h ) for allowing said drain passage and a portion of said connecting passage extending from said changeover valve on a tilting piston side to communicate with each other midway in a stroke in which said changeover valve is changed over from the high-speed position to the low-speed position; and  
       a second narrow passage ( 82 ,  65   g ,  82 ) for allowing the portion of said connecting passage extending from said changeover valve on the tilting piston side and a portion of said communicating passage extending from said changeover valve on said selector valve side to communicate with each other midway in a stroke in which said changeover valve is changed over from the high-speed position to the low-speed position.  
     
     
       2. The drive circuit for a fluid motor according to  claim 1 , wherein said first narrow passage is formed such that a flow passage area thereof becomes larger as said changeover valve approaches the low-speed position, while said second narrow passage is formed such that a flow passage area thereof becomes smaller as said changeover valve approaches the low-speed position. 
     
     
       3. The drive circuit for a fluid motor according to  claim 1 , wherein said first and said second narrow passages are notches formed in an outer periphery of a spool of said changeover valve in such a manner as to extend in an axial direction. 
     
     
       4. The drive circuit for a fluid motor according to  claim 1 , wherein said first and said second narrow passages are narrow grooves formed in an outer periphery of a spool of said changeover valve or in an inner periphery of a spool chamber for accommodating said changeover valve in such a manner as to extend in an axial direction. 
     
     
       5. The drive circuit for a fluid motor according to  claim 1 , wherein said second pilot passage is branched off from said connecting passage extending from said changeover valve on the selector valve side so as to introduce into said changeover valve the high-pressure fluid obtained by said selector valve.